1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus in which an Automatic Retransmission reQuest (ARQ) transmitting (Tx) entity recognizes a transmission failure that an ARQ receiving (Rx) entity has failed to recognize, and automatically retransmits a data packet.
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a 3rd Generation (3G) asynchronous mobile communication system that is based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), both of which are European mobile communication systems, and uses Wideband Code Division Multiple Access (WCDMA).
In 3rd Generation Partnership Project (3GPP), which is in charge of UMTS standardization, Long Term Evolution (LTE) is being discussed as the next generation mobile communication system of the UMTS system. LTE, aimed deployment in around 2010, is a technology for implementing high-speed packet based communication at about 100 Mbps. As such, several schemes are under discussion, and the schemes under discussion include one scheme of reducing the number of nodes located in a communication path by simplifying the network configuration, and another scheme of maximally approaching wireless protocols to wireless channels.
FIG. 1 illustrates an exemplary configuration of an Evolved UMTS mobile communication system.
In FIG. 1, a configuration of Evolved UMTS Radio Access Networks (E-UTRANs or E-RANs) 110 and 112 is simplified to a 2-node configuration of Evolved Node Bs (ENBs or Node Bs) 120, 122, 124, 126 and 128, and anchor nodes 130 and 132. A User Equipment (UE) 101 accesses an Internet Protocol (IP) network by the E-RAN 135 and 140. The anchor nodes 130 and 132 are denoted by Evolved Gateway GPRS Serving Node (EGGSN). A User Equipment (UE) 101 accesses an Internet Protocol (IP) network by the E-RANs 110 and 112.
The ENBs 120 to 128, nodes corresponding to the existing Node Bs of the UMTS system, are connected to the UE 101 over a wireless channel. Compared with the existing Node Bs, the ENBs 120 to 128 perform more complex functions. In LTE, all user traffics, including real-time services such as Voice over IP (VoIP) service, are serviced over a shared channel. This means that there is a need for an apparatus for collecting status information of UEs and performing scheduling depending thereon. The scheduling is managed by the ENBs 120 to 128.
In LTE, like in High Speed Downlink Packet Access (HSDPA) or Enhanced Uplink Dedicated Channel (EDCH), Hybrid ARQ (HARQ) is performed between the ENBs 120 to 128 and the UE 101. However, with use of only the HARQ, LTE cannot satisfy various Quality of Service (QoS) requirements. Therefore, a separate ARQ (or outer-ARQ) can be performed in an upper layer, and the outer-ARQ is also performed between the ENBs 120 to 128 and the UE 101.
In this case, LTE will use Orthogonal Frequency Division Multiplexing (OFDM) as wireless access technology in a 20-MHz bandwidth in order to realize a data rate of a maximum of 100 Mbps. In addition, LTE will employ Adaptive Modulation & Coding (AMC) that determines a modulation scheme and a channel coding rate according to channel status of UEs. Many mobile communication systems now under discussion, including LTE, use both HARQ and ARQ as an error correction technique.
HARQ is a technique for soft-combining previously received data with retransmitted data without discarding the previously received data, thereby increasing a reception success rate. More specifically, an HARQ receiving (Rx) entity determines presence/absence of error in a received packet, and sends an Acknowledged (ARQ ACK) signal or a Non-Acknowledged (ARQ NACK) signal to an HARQ transmitting (Tx) entity according to the presence/absence of error. The HARQ Tx entity performs retransmission of the HARQ packet or transmission of a new HARQ packet according to the HARQ ACK/NACK signal. The HARQ Rx entity soft-combines a retransmitted packet with a previously received packet, thereby reducing an error rate.
However, ARQ, a technique for checking a sequence number of a received packet and sending a retransmission request for a missing packet using a status report, does not soft-combine a previously received packet with a retransmitted packet.
Because both ARQ and HARQ have a function of recovering erroneous packets, it is considered that there is no need to perform ARQ and HARQ together. However, because a sufficiently low packet error ratio cannot be obtained with use of only HARQ, ARQ and HARQ should be performed together in most packet services. In HARQ, because an HARQ ACK/NACK signal is a 1-bit feedback signal, it is difficult to reduce an error rate of the feedback signal through channel coding. Therefore, it is hard to achieve a low packet error rate with use of only HARQ. If an HARQ NACK signal is misrecognized as an HARQ ACK signal, the corresponding packet is completely lost in the HARQ layer. Therefore, reliability of the HARQ ACK/NACK signal acts as an important factor in determining a packet error ratio in the HARQ layer.
An ARQ operation determines an ARQ packet corresponding to a gap among sequence numbers, as a missing packet (or lost packet), and issues a retransmission request for the missing packet. However, when the last packet is lost, there is no gap among the sequence numbers. Therefore, the ARQ Rx entity cannot issue a retransmission request for the last packet. Accordingly, there is a need for a technique for performing fast retransmission even when the last packet is lost in the mobile communication system where both ARQ and HARQ operate together.